Zinc and cadmium electroplating



United States Patent C) 3,294,655 ZINC AND CADMIUM ELECTROPLATING Leslie E. Lancy, Ellwood City, Pa., assignor to Lancy Laboratories, Inc., Zelienople, Pa., a corporation of Pennsylvania No Drawing. Filed Nov. 4, 1963, Ser. No. 321,358 18 Claims. (Cl. 204-50) This invention relates to the electroplating of metals in the nature of zinc and cadmium and particularly, to an improved electroplating solution and utilization thereof.

A phase of my invention deals with the prevention of so-called aging elfects in an electroplating solution that after a period of time, make the solution less efiicient and effective and require its ultimate disposal. I have found in the electroplating of metals of the class or nature of zinc and cadmium from a cyanide type plating solution that, during the use of the plating solution, changes occur due to aging and that this aging is caused by a build-up of foreign materials, such as may be dragged-in by the work pieces or the water used or that may be produced by the process, itself. In the present invention I have been particularly concerned with aging that is caused by a build-up of sodium or potassium carbonates in the solution from the decomposition of sodium or potassium cyanides, either as a result of oxygen released from the anode or from absorption of air as well as from the absorption of carbon dioxide by the caustic soda constituent from the air.

Another significant troublesome constituent of an aged cyanide plating solution results from the formation of soluble iron cyanide compounds. This is due to the dissolution by the cyanide of basis metal salts, such as iron sulfate or chloride on the surface or in the pores of the work piece (cathode) when entering the solution, and the slow dissolution of the basis metal from insoluble anode materials, such as steel wire anode containers, tank walls, etc. In attempting to find a solution to aging caused by the build-up of highly soluble sodium or potassium carbonates and soluble sodium or potassium iron cyanides, I found that due to the inevitable slow but progressive build-up, that the problem of maintaining good plating efiiciency from day to day and of maintaining overall performance becomes gradually more diflicult as the solution became older.

It has been customary, in plating baths for metals of the type involved, to utilize in the solution a cyanide of the respective plating metal and an alkali metal in the nature of sodium or potassium. In addition, an alkali metal in the form of a hydroxide, such as caustic soda is also employed. A typical cyanide plating solution for zinc may have a content of 8 to 10.6 ounces per gallon of zinc cyanide and of 10.5 to 15 ounces per gallon of caustic soda. On analysis, the solution will show a content of metallic zinc of 4.5 to 6 ounces per gallon, with total cyanide expressed as sodium cyanide of 12.3 to 14 ounces per gallon, and caustic soda in the amount indicated above.

A typical cyanide plating solution for cadmium may have a content of 1.5 to 3 ounces per gallon of cadmium metal, a total cyanide content expressed as sodium cyanide of 7.5 to 18 ounces per gallon, and a caustic soda content of 1.5 to 5 ounces per gallon.

In the above conventional or typical cyanide plating solution examples, the cadmium or zinc concentration 3,294,655 Patented Dec. 27, 1966 in each case can be varied depending on requirements. In this connection, a lower rate of deposition will allow a lower concentration of zinc or cadmium salts, sometimes as low as two ounces per gallon expressed as zinc metal and as low as one ounce per gallon expressed as cadmium metal. A higher zinc or cadmium concentration will favor higher deposition rate requirements. The cyanide concentration for analytical purposes is usually expressed as total sodium cyanide, but is actually zinc or respectivelly cadmium cyanide and free sodium cyanide. Cyanide is the main complexing agent of the zinc or cadmium salts and is also needed to dissolve fresh zinc or cadmium from the anode as the solution is depleated of the plating metal during the cathode deposition process. Those well versed in the art of plating usually establish the cyanide requirements for a particular plating solution depending upon the zinc or cadmium concentration, and will recommend a ratio range of total sodium cyanide to zinc of 2/1 to 3/1 or to cadmium of 5/1 to 6/1. The lower ratios will aid in a higher deposition rate, providing a higher cathode current efiiciency in the electrodeposition process. Cathode effieiency means the percentage of current resulting in metal deposition as compared to a theoretical maximum of Higher ratios, while depressing current efficiencies, will aid anode dissolution and plating on recessed areas on the work pieces which is called improving the throwing power.

The caustic soda constituents of the plating solution serves the function of providing an additional complexing agent for the dissolved zinc or cadmium salts, especially with lower ratios. This also aids the dissolution of the zinc or cadmium on the anode. In addition, the caustic soda will improve the conductivity of the plating solution, and higher caustic soda concentrations will permit the passing of the required current at a lower impressed potential, all other factors remaining the same.

In both types of plating solutions or baths, brightening agents are usually used With the main constituents. These may be inorganic, such as sodium sulfide or other sulfides in the case of a zinc solution; in the case of either a zinc or cadmium solution, they may be metallic salts, such as of molybdenum, co'balt, nickel, etc., usually within a less than one ounce per gallon range, and may also be used with inorganic salts and/ or small quantities of organic addition agents, such as dextrin, vanillin, gum ara'bic, 'aldehydes, etc.

The problem of aging in both types of solutions appears to be the same and is of the nature previously described.

It has thus been an object of my invent-ion to provide a solution to the problem of deterioration or aging of plating solutions with usage and particularly, from the standpoint of a buildsup of undesirable highly soluble compounds;

Another object of my invention has been to devise a new approach to plating solutions which will make use of a significant or appreciable minimum amount of soluble lithium compounds Another object of my invention has been to provide an improved appearance of the plating metal in the sense of a more uniform deposition and a finer grain deposit;

A further object has 'been to provide a more uniform anode dissolution and higher resistance to polarization due to high anode current densities;

A further object has been to provide a better and more uniform maintenance of a desired electroplating current flow -or amperage, employing a given normal desire potential;

A still further object has been to provide a plating solution that has a great resistance against the usual effects of aging and thus, that will have a greatly increased effective operating life;

These and other objects of my invention will appear to those skilled in the art from the illustrated embodiments thereof and the appended claims.

Heretofore, as above indicated, it has been customary in cyanide plating baths for metals, such as cadmium and zinc, to use alkali metal compounds of the alkali metals sodium and potassium and particularly sodium. I have discovered that the problem involved may be solved by the use of a significant minimum amount of soluble lithium metal compound in the plating solution. Although the lithium compound may constitute the sole alkali metal used in the aqueous electroplating solution or bath, it will preferably constitute a definite minimum significant proportioned amount of the alkali metal of the bath. Although the use of lithium salts, for example in the nature of lithium hydroxide or cyanide, has been heretofore avoided in significant proportions in plating baths, possibly due to the higher cost of lithium metal, namely about $.58 per pound for lithium hydroxide (40.7% OH), as compared to $.06 /2 per pound for sodium hydroxide (42.5% OH), I have discovered that although the cost per pound is almost ten times greater, its use in significant amounts produces new and improved results that more than ofliset such increased cost. Also, a conventional alkali metal, such as sodium, can be used in a cadmium plating solution to provide substantially about 95% of the alkali metal requirements of the bath While, at the same time, insuring the improved results of my invention by the use of a substantial or significant amount of lithium metal which will constitute substantially about 5% of the alkali metal requirements of the solution; see Example 1. In the case of a zinc solution, a conventional alkali metal, such as sodium, may be employed to provide substantially about 95% of the requirements of the solution and the lithium metal to provide substantially about 5% to assure the improved results of my invention; see Example 2.

EXAMPLE 1 15 oz./ gal. NaCN: Oz./ gal.

EXAMPLE 2 oz./gal. NaCN, 47% Na 4.7 10 oz./gal. NaOH, 57.5% Na 5.75

In accomplishing the improved results of my invention, the cyanide aqueous solution should contain a suflicent soluble lithium compound or compounds to form substantially or relatively insoluble lithium carbonates and complex metal cyanides in the solution in the nature of lithium iron cyanides, during the useful life of the solution as employed in its electroplating and, as distinguished from the formation of readily soluble sodium or potas sium carbonates and sodium or potassium complex cyanide compounds, such as sodium or potassium iron cyanides. The soluble lithium compound should be added or be present in the solution in a significant amount which, for practical purposes from the standpoint of both zinc and cadmium electroplating solutions, will be a minimum of about 5% by weight. In other words, using such a minimum of the soluble lithium compound, the remaining alkali metal requirements of the solution as, for example, supplied by alkali metals of the class of sodium or potassium, will be in the amount of about by weight. It is thus apparent that the aqueous cyanide treating solutions incorporating my invention may utilize conventional quantities of alkali metal cyanides and alkali metal hydroxides, but'that the soluble lithium compound must be in a significant percentage or amount in supplying a portion or all of the alkali metal requirements of the solution.

By way of example, three stock solutions were made up with comparable basic compositions to allow a comparison of typical zinc cyanide plating solutions, in which solution A has all sodium salts, solution B has all lithium salts, and solution C has mixed sodium and lithium salts wherein the lithium compounds were maintained at a significant level, such that they constitute a major constituent of the plating solution make-up. To properly compare the various solutions, the constituents are, from an analytical standpoint, somewhat differently expressed as explained earlier, in that the zinc content of the solutions was uniformly analyzed for zinc metal. The total cyanide concentration is expressed as CN- rather than as sodium cyanide, and a comparable ratio of zinc to NaCN of 1/2.5 to 2.6 is expressed as Zn+ to ON of 1/1.33 to 1.35. The caustic concentration is expressed as OH, indicating the significant hydroxide ion, regardless of the alkali metal employed. In the three zinc solutions under comparison, the zinc metal, the total CN and the total OH were held constant, irrespective of whether the solution was made up of sodium or lithium cyanide or, for the hydroxide concentration, sodium or lithium salts were added.

Solution A content Zinc++ to CN- ratio=1 to 1.34.

Solution C content sodium-lithium mixed system Oz. er al. Zinc metal -F 3 :98 NaCN 10.04 LiOH.H O 10.41 CN- 5.32 OH 4.25

Zinc++ to CN- ratio=1 to 1.336.

Also, by way of example, I prepared three stock cadmium plating solutions having approximately the same quantity contents of cadmium metal, cyanide and hydroxide. Solution A contains all sodium salts, solution B contains all lithium salts using lithium cyanide and hydroxide for the make-up, and solution C contains a mixture of sodium and lithium salts using sodium cyanide and lithium hydroxide for the make-up. The total cyanide concentration is expressed as CN- irrespective of the type of alkali metal used, and the alkalinity is expressed as OH- irrespective of the particular alkali metal used.

02. per gal. Cadmium metal 2 NaCn 15.21

NaOH 5.45

OH 2.32 Ratio of Cd++ to CN-=1 to 4.031.

Solution B content lithium system Oz. per gal. Cadmium metal 1.985 LiCn 10.06

LiOH-H O 4.24 CN 7.94

Ratio of Cd++ to CN*=1 to 4.

Solution C content sodium-lithium mixed system Oz. per gal. Cadmium metal 2.02 NaCn 15.40 LiOH-H O 4.70 CN- 8.16 OH- 1.91

Ratio of Cd++ to CN-=1 to 4.04.

The aging of the six solutions was simulated by adding .1 ounce per gallon of ferrous sulfate to form ferrocyanides in the amount of approximately .187 ounce per gallon of Na Fe(CN) -10I-1 O and also by adding sodium carbonate in various stages up to ten ounces per gallon to the sodium solutions systems A and A and to the sodium-lithium mixed systems C and C For the lithium systems B and B CO gas was bubbled therein until saturation was attained.

Comparing the freshly made up solutions, a definite improvement was indicated as to the appearance of the cathode deposits and as to the potential needed to impress a desired current in solutions B and C and B and C over solutions A and A The differences were even more pronounced from the standpoint of the effect of aging. For observation and comparison, a 250 ml. Hull cell was used which is a well known device for comparing the performance of various plating solutions. In all six solutions, the same organic brightening agents were used and plating was effected of two ampere Hull cell panels at a room temperature of 80 F.

With reference to solutions A, B and C, plating was accomplished at 2 amperes for minute at a room temperature of 80 F. Solution A started to plate at 2.2 volts, but the potential had to be raised to 4 volts to maintain the 2 amperes desired during the plating period. As to solution B, under identical conditions, only two volts were required initially to maintain the two amperage current and no change was necessary as to the voltage throughout the completion of the test. In solution C, two volts were applied at the beginning of the test and was maintained throughout to provide the desired 2 amperes current.

Solution A started to plate at 2.65 volts, but the voltage had to be raised to 2.9 volts to maintain the desired 2 amperes on the Hull cell panel during the ten minute plating test. Excessive gassing was noticed on the cathode panel and also on the cadmium anode, indicating both polarization and reduced efliciency on the electrodes.

Solution B required only 2.4 volts to maintain 2 amperes of plating current without the need of increasing the potential during the ten minute plating test. No polarization was noticed on the anode and gassing occurred only on the high current density end of the cathode panel.

With reference to solution C 2.4 volts potential was required to maintain the 2 amperes desired current dens ty and it was found that this could be reduced to 2.2 volts 6 before the ten minute plating test period was completed. No gassing was noticed on the anode, indicating anode efiiciency. Gas evolution on the cathode panel was restricted to the high current density end.

The plating cathode panel of solutions B and C and B and C were more uniformly bright with finer grain deposits than solutions A and A With reference to solutions B and C, and also solutions B and C there was considerably [less hydrogen gas evolution at the high current density area of the Hull cell panel, indicating a far better current efiiciency. There was also no gassing on the anodes of the latter solutions, while using the solutions A and A a slight amount of gassing occurred, indicating a noticeable amount of oxygen evolution.

My operation of an automatic zinc plating process with a plating solution of 8000 gallons capacity, using an average of 9000 amperes of direct current for deposition over a period of ten months, nearly constantly on a 6 day, 24 hour basis, clearly indicated the unpredictable improvements due to significant lithium salt additions.

With reference to the results attained employing my invention in both cadmium and, zinc plating solutions, there is a definitely improved appearance of the deposit from the standpoint of uniformity of deposition and a much finer grain. Also, I found that the solutions have great resistance to the usual effects of aging from the standpoint of impurity build-up of the type that is dragged into the solution on the work pieces, from the standpoint of contamination from the water used, and also from the standpoint of decomposition of chemicals used in the solution composition. There is a more uniform anode dissolution and resistance to polarization due to the ability to maintain high anode current density, and finally, there is a better and more uniform maintenance of the desired amperage with normal electrical potentials employed.

In zinc plating, I have found that three ounces per gallon of lithium hydroxide monohydrate (LiOH-H O) which is the equivalent of .5 ounce of lithium metal per gallon of zinc plating solution or of 2.5 ounces per gallon of lithium cyanide, is critical and required to satisfy the minimum benefits of my invention. The upper limit would be the use of all lithium compounds for satisfying the alkali metal hydroxide and cyanide requirements of the plating solution.

With regard to cadmium plating, I have discovered that 2 ounces per gallon of lithium hydroxide mono hydrate (LiOH-H O) is the minimum requirement. This is equivalent to .3 32 ounce per gallon of lithium and the equivalent of a minimum content of 1.66 ounces per gallon of lithium cyanide. The minimum significant lithium content was found to be critical for achieving minimum benefits of my invention. It is apparent that the upper limit is the amount of lithium required to furnish the total alkali metal cyanide and hydroxide requirements of the plating solution with the complete exclusion of sodium and potassium compounds.

In both types of solutions, the main constituents are the respective cadmium or zinc metal ions, the lithium ions are also included in a significant quantity, as. well as the sodium or potassium ions unles the formulation is with the exclusion of the latter in an all lithium bath. Although I have not been able to definitely determine why a significant quantity of lithium metal will provide greatly improved results, I have found that the lithium content should be sufficient that carbonates formed are lithium carbonates rather than sodium or potassium carbonates, and that alkali metal ferrocyanides formed should have a lithium rather than a sodium or potassium content. In this connection, the lithium acts preferentially in picking-up contaminants in the solution and produces compounds that are relatively insoluble in the solution, as compared to compounds produced by sodium or potassium which are highly soluble. It is thus apparent that the minimum significant content of lithium metal must be sufficient to provide the content necessary for forming carbonates and alkali metal ferrocyanides, etc., within the solution. The percentages herein set forth are by weight.

I have found that as to the exemplary solutions A,- A B, B C and C and in providing normal amounts of alkali metal cyanides and hydroxides in electroplating solutions, that sodium and potassium compounds are interchangeable in substantially equal quantities. Also, although from a standpoint of economy of alkali metal additions, it is preferable to use a quantity of the lithium metal compounds to bring the solution up to a normal total content of alkali metal cyanides and hydroxides, that the improved results of my invention can also be attained by adding a soluble lithium metal compound in an amount over and above such a normal total content, provided that the lithium compound is added in a significant quantity sufficient to prevent aging of the solution from the standpoint of substantially preventing the formation of soluble alkali metal carbonates and soluble iron cyanides in the solution.

Although, for the :purpose of illustration, I have set forth examples of my invention and particular applications thereof, it will be apparent to those skilled in the art that various modifications and adaptations may be made without departing from its spirit and scope as indicated by the appended claims.

What I claim is:

1. A method of imparting an electrolytic metal deposit of the class consisting of zinc and cadmium on the surface of a base metal which comprises, applying an electroplating current to an aqueous solution containing a metal cyanide of the corresponding metal to be plated and containing normal quantities of alkali metal cyanides and hydroxides, and supplying a soluble lithium metal compound to the solution in a significant quantity sufficient to prevent aging of the solution from the standpoint of substantially preventing the formation of soluble alkali metal carbonates and soluble iron cyanides therein.

2. A method as defined in claim 1 wherein soluble lithium compounds are supplied to the solution in suffi cient quantities to only form substantially insoluble lithium carbonates and iron cyanides during the aging of the solution.

3. A method of electroplating a metal of the class consisting of zinc and cadmium from a corresponding metal cyanide solution containing alkali metal cyanide and alkali metal hydroxide which comprises, supplying lithium metal to the solution in the form of a soluble lithium compound of a quantity to provide the equivalent of at least 2.5 ounces of lithium hydroxide monohydrate per gallon for the electroplating of zinc and at least 2 ounces of lithium hydroxide monohydrate per gallon for the electroplating of cadmium, and then electroplating the metal from the solution.

4. A method as defined in claim 3 wherein the lithium is supplied to the solution in the form of a compound of the class consisting of lithium cyanide and lithium hydroxide.

5. A method as defined in claim 4 wherein all of the alkali metal cyanide and alkali metal hydroxide requirements of the solution are supplied thereto by soluble lithium compounds.

6. A method as defined in claim 4 wherein a significant but minor proportion of the total alkali metal cyanide and alkali metal hydroxide requirements of the solution are supplied by soluble lithium compounds, and a major proportion of the total alkali metal cyanide and hydroxide requirements of the solution are supplied by cyanide and hydroxide compounds of the class consisting of sodium and potassium.

7. A method of electroplating a metal of the class consisting of zinc and cadmium from a corresponding metal cyanide solution containing alkali metal cyanide and alkali metal hydroxide compounds which comprises, supplying lithium metal to the solution in the form of a soluble lithium compound in a significant amount sufficient to provide a minimum of about 5% of the alkali metal requirements of the solution, and then electroplating the metal from the solution.

8. A method of electroplating a metal of the class consisting of zinc and cadmium from a corresponding metal cyanide solution containing alkali metal cyanide and alkali metal hydroxide compounds which comprises, supplying lithium metal to the solution in the form of a soluble lithium compound in a significant amount sufficient to provide a minimum of about 5% of the alkali metal requirements of the solution, supplying an alkali metal of the class consisting of sodium and potassium in a sufficient amount to provide remaining alkali metal requirements of the solution, and then electroplating the metal from the solution.

9. A method as defined in claim 8 wherein substantially 5% by weight of the alkali metal content of the solution is supplied by the lithium metal and substantially by weight of the alkali metal content of the solution is supplied by a metal of the class consisting of sodium and potassium.

10. A method of electroplating a metal of the class consisting of Zinc and cadmium from a corresponding metal cyanide solution containing alkali metal cyanide and alkali metal hydroxide which comprises, supplying lithium metal to the solution in the form of a lithium compound of a quantity to provide the equivalent of at least 3 ounces of lithium hydroxide monohydrate per gallon for the electroplating of zinc and of at least 2 ounces of lithium hydroxide monohydrate per gallon for the electroplating of cadmium, and proportioning the quantity of the lithium compound above the specified minimum amounts to satisfy the total alkali metal cyanide and hydroxide requirements of the solution for the electroplating operation.

11. A method as defined in claim 10 wherein the lithium is supplied to the solution in the form of a lithium cyanide compound.

12. A method as defined in claim 10 wherein the lithium is supplied to the solution in the form of a lithium hydroxide compound.

13. A method as defined in claim 10 wherein the lithium is supplied to the solution in the form of lithium cyanide and hydroxide compounds.

14. An aqueous solution for electroplating metals of the class consisting of zinc and cadmium that contains conventional amounts of a corresponding metal cyanide and of alkali metal cyanides and hydroxides, the alkali metal content representing a sufiicient significant quantity of a soluble lithium compound to substantially inhibit the formation of readily soluble alkali metal carbonates in the solution and to maintain a substantially constant desired electroplating current flow in the solution by the application of substantially the same potential throughout the aging of the solution.

15. An aqueous solution for electroplating metals of the class consisting of zinc and cadmium that contains conventional amounts of a corresponding metal cyanide and of alkali metal cyanides and hydroxides wherein, a minimum of about 5% by weight of the alkali metal cyanide and hydroxide content is provided by a soluble lithium compound.

16. An aqueous solution as defined in claim 15 wherein all of the alkali metal cyanide and hydroxide content of the solution is provided by the soluble lithium compound. I

17. An aqueous solution as defined in claim 15 wherein about 95 by weight of the alkali metal cyanide and hydroxide content of the solution is provided by alkali metals of the class consisting of sodium and potassium.

18. An aqueous solution as defined in claim 15 wherein the lithium content thereof is sufficient to convert its carbonate content into lithium carbonate and its metal cyanide content into a lithium metal cyanide.

References Cited by the Examiner UNITED STATES PATENTS 2,164,924 7/1939 Hull 204-50 2,443,600 6/1948 Chester 204-40 2,787,590- 4/1957 Rinker 2o4 5o X 10 10 2,858,257 10/1958 Ceresa et al. 204-50 X 2,861,927 11/1958 Ceresa et a1. 204 50 X OTHER REFERENCES Gray, A. G.: Modern Electroplating, pages 118- 119, 1953.

JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

1. A METHOD OF IMPARTING AN ELECTROLYTIC METAL DEPOSIT OF THW CLASS CONSISTING OF ZINC AND CADMIUM ON THE SURFACE OF A BASE METAL WHICH COMPRISES, APPLYING AN ELECTROPLATING CURRENT TO AN AQUEOUS SOLUTION CONTAINING A METAL CYANIDE OF THE CORRESPONDING METAL TO BE PLATED AND CONTAINING NORMAL QUANTITIES OF ALKALI METAL CYANIDES AND HYDROXIDES, AND SUPPLYING A SOLUBLE LITHIUM METAL COMPOUND TO THE SOLUTION IN A SIGNIFICANT QUANTITY SUFFICIENT TO PREVENT AGING OF THE SOLUTION FROM THE STANDPOINT OF SUBSTANTIALLY PREVENTING THE FORMATION OF SOLUBLE ALKALI METAL CARBONATES AND SOLUBLE IRON CYANIDES THEREIN. 